1. Field of the Invention
The present invention relates to a fixing device for thermally fixing a developer image transferred onto a recording sheet.
2. Description of Related Art
In a fixing device provided in an image-forming apparatus, it is generally known that a sheet of paper (or recording sheet) tends to curve along the peripheral surface of a heating roller or a pressure roller, which phenomenon is sometimes called “curl” of the sheet. The curl of the sheet would result in a sheet jam or a disturbed order of sheets ejected onto a sheet output tray. With this in view, a fixing device of a particular type has been proposed (for example, see patent document 1 listed below), in which a pressure roller is configured to be movable relative to a heating roller so that the position of the pressure roller is adjusted relative to the heating roller in accordance with the sheet curling condition to thereby correct the curl of a sheet.
The fixing device has an interrupter element such as a thermostat or a thermal fuse provided across the heating roller from the pressure roller, the interrupter element being configured to sense a surface temperature of the heating roller so as to interrupt the flow of electrical current to a heat source of the heating roller when the temperature is not lower than a predetermined threshold value. When the surface temperature of the heating roller increases beyond a fixing temperature and a plastic bushing for the heating roller is molten, the heating roller pressed by the pressure roller comes closer to the interrupter element. In this way, the temperature sensed at the interrupter element becomes the predetermined threshold value or higher more swiftly, and thus the flow of electrical current to the heat source is swiftly interrupted so that the increase in the temperature of the heating roller can be swiftly stopped. See patent document 2 listed below, for example.